1. Field of the Invention
The present invention relates to a braking system for use in an automotive vehicle and particularly to a hydraulic braking system having a master cylinder and a hydraulic booster which outputs or transmits hydraulic power pressure, supplied from a power source, in response to the depression of a brake pedal.
2. Description of the Related Art
In conventional service braking systems for automotive vehicles, a plurality of hydraulic circuits are provided for connecting wheel brake cylinders mounted on road wheels of the vehicle with a hydraulic braking pressure generator such as a master cylinder. In the event that one of the hydraulic circuits is broken or inoperative, the braking operation is achieved by the remainder of the hydraulic circuits. In general, a tandem master cylinder is used in conventional dual circuit brake systems.
In order to reduce the force required to operate a brake pedal during the braking operation, the hydraulic braking system is provided with a servo unit which is referred to as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator, such as the master cylinder, by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. For example, the Japanese patent Laid-open publication No. 59-209948 discloses a system connecting the hydraulic booster to the tandem master cylinder which will operate as an ordinary tandem master cylinder when the hydraulic booster is not operative.
It has been proposed to employ the hydraulic booster as a dynamic hydraulic pressure generator in addition to the master cylinder. In other words, a hydraulic pressure boosted by the booster (hereinafter referred to as boost pressure) in response to the depression of the brake pedal is applied directly to a hydraulic circuit. For example, as shown in Japanese Patent Laid-open publication No. 59-227552, boost pressure from the hydraulic booster is applied to rear wheel brake cylinders in a front-rear dual hydraulic circuit system in order to reduce the travel stroke of the brake pedal.
Further, it has been proposed to connect one of the pressure chambers of the tandem master cylinder with the wheel cylinders of one of the hydraulic circuits and to connect the boost pressure of the hydraulic booster to the wheel cylinders of the other hydraulic circuit through the other pressure chamber of the tandem master cylinder. Such a hydraulic braking system is disclosed in U.S. Pat. No. 4,800,724. According to this system, various effects such as shortening the travel stroke of the brake pedal and so on are obtained.
In the above described prior art, (Japanese Patent Laid-open publication No. 59-227552), however, since the boost pressure of the hydraulic booster, i.e., the dynamic hydraulic pressure is directly supplied to the rear wheel cylinders, the travel stroke of the brake pedal is reduced when generating the hydraulic pressure to be provided to the rear wheel cylinders. However, the hydraulic pressure to the front wheel cylinders is not generated until a normally open valve operator for a piston causes the piston to operate and interrupt the fluid communication between a fluid chamber and a pressure chamber. Therefore, an initial idle stroke exists in the brake pedal operation and the travel stroke of the brake pedal cannot be reduced. In U.S. Pat. No. 4,800,724, the boost pressure, i.e., the dynamic hydraulic pressure is directly supplied to the wheel cylinders of the other hydraulic circuit via a second fluid chamber and a second pressure chamber except during abnormal operating conditions. Accordingly, the stroke of the brake pedal generating the hydraulic pressure for the wheel cylinders of the other hydraulic circuit is reduced. However, a first pressure chamber is in communication with a reservoir through a compensating port until a piston is moved so as to close the fluid communication between the reservoir and the first pressure chamber due to a cup seal provided on the piston. Due to this arrangement, an initial idle stroke exists in the brake pedal operation.